Time-variable gravity potential components for optical clock comparisons and the definition of international time scales

The latest generation of optical atomic clocks is approaching the level of one part in 1018 in terms of frequency stability and uncertainty. For clock comparisons and the definition of international time scales, a relativistic redshift effect of the clock frequencies has to be taken into account at a corresponding uncertainty level of about 0.1 m(2) s(-2) and 0.01 m in terms of gravity potential and height, respectively. Besides the predominant static part of the gravity potential, temporal variations must be considered in order to avoid systematic frequency shifts. Time-variable gravity potential components induced by tides and non-tidal mass redistributions are investigated with regard to the level of one part in 10(18). The magnitudes and dominant time periods of the individual gravity potential contributions are investigated globally and for specific laboratory sites together with the related uncertainty estimates. The basics of the computation methods are presented along with the applied models, data sets and software. Solid Earth tides contribute by far the most dominant signal with a global maximum amplitude of 4.2 m(2) s(-2) for the potential and a range (maximum-to-minimum) of up to 1.3 and 10.0 m(2) s(-2) in terms of potential differences between specific laboratories over continental and intercontinental scales, respectively. Amplitudes of the ocean tidal loading potential can amount up to 1.25 m(2) s(-2), while the range of the potential between specific laboratories is 0.3 and 1.1 m(2) s(-2) over continental and intercontinental scales, respectively. These are the only two contributors being relevant at a 10(-17) level. However, several other time-variable potential effects can particularly affect clock comparisons at the 10(-18) level. Besides solid Earth pole tides, these are non-tidal mass redistributions in the atmosphere, the oceans and the continental water storage.